The invention pertains to apparatus and methods for communicating signals between processors in multi-processor systems. More particularly, the invention pertains to such systems wherein the processors communicate with one another via a communications medium as in a local area network.
Communications circuitry for use in multi-processor systems dedicated to monitoring or supervising regions is known. One example is disclosed in Tice et al U.S. Pat. No. 4,916,432 entitled Smoke and Fire Detection System Communication. Another is disclosed in Tice U.S. Pat. No. 5,525,962 entitled Communication System and Method. Both of the noted patents are assigned to the assignee hereof and are hereby incorporated herein by reference.
While known systems are useful and have been effective, it would be desirable to be able to more completely separate data from clock signals during the communication process. Further, it would be desirable to be able to provide a substantially collision free communication environment. Such an environment would be useful in supervision or alarm systems as well as in general purpose local area networks.
A communications apparatus utilizes multi-polarity, representations for clock and data pulses. Clock pulses are transmitted from a source in a first polarity, in a communications medium as voltage pulses. The source transmits clock pulses with a low output impedance. In-between clock pulses, the source switches to a high output impedance.
At least some of the data pulses are transmitted in a second polarity, on the medium, as voltage pulses. Most of the data pulses are bracketed by pairs of clock pulses.
In one aspect data pulses, for example representing a logical xe2x80x9conexe2x80x9d, can be transmitted as substantially constant width pulses with logical xe2x80x9czeroxe2x80x9d being represented by absence of a pulse. Alternately, data can be represented as variable width voltage pulses. A logical xe2x80x9conexe2x80x9d can be transmitted with a first width and a logical xe2x80x9czeroxe2x80x9d transmitted with another width.
In one aspect, where the source corresponds to a common control element, energy can be supplied to a plurality of spaced part units coupled to the medium, at least, when the clock pulses are being generated by the control element. In this embodiment, data can be generated by the control element, with the second polarity or by another of the units coupled to the medium.
In yet another aspect, the control element can provide framing signals for messages along with the clock pulses to synchronize communications on the line.
Further, since the clock signals and the data signals are transmitted with different polarities relative to the medium signal-to-noise characteristics are improved. For example, if the first polarity is opposite the second polarity, the respective detection thresholds can be spaced further apart from one another, i.e., +2.5 volts and xe2x88x922.5 volts, respectively. Finally, the polarity of a particular pulse also identifies the type of information, clock or data, represented by the pulse.
Other advantages include:
The clocking waveform and the device data waveform will never occur at the same time. This makes it possible to implement a lockout design in the detection circuit that will tend to prevent a false clock or data detection from xe2x80x9cringingxe2x80x9d on the line during the driving of the clock and data voltage waveforms.
A device wired backwards will not short out the communication wiring. The system can determine which devices are wired backwards without interference with the devices that are wired correctly. (The system may be able to communicate to such devices without having to correct the wiring under certain conditions).
The ability to differentiate from a low impedance (causing a low voltage on the line) and data.
In order to minimize xe2x80x9cringing and other distortionsxe2x80x9d on the wiring during communications, an adjustable waveform shape can be driven from the power source for clocking. The xe2x80x9cslew ratexe2x80x9d or transition rate of the voltage from one level to another can be adjusted to compensate for various wiring configuration. This will tend to minimize distortion of the waveform during communications. This waveform adjustment will be a function of:
propagation times for the signals on the wiring due to lengths and characteristic impedances of the wires,
errors occurring in the communications which is monitored by every device on the loop, and
waveform analysis at a central point, most likely the power source for clocking.
In yet another aspect, bytes of data can be transmitted with single intervening clock signals. Alternately, transmission can be implemented with only a single synchronizing signal followed by a string of data such as one or more bytes.
Collision free communications can be accomplished by having the devices monitor the communication line voltages while they are transmitting. Any mismatch in voltage causes a transmitting device to drop off the line and wait for the next access period to start transmitting again.
Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings.